Microfilming, as a documentary reproduction process, is in widespread and increasing use. A microfilm copy may be defined as a transparent photocopy at a reduction sufficient that optical enlargement is required for normal reading and with resolving power sufficient for accurate recording of textual and tonal detail. Normal reductions are from 12 to 42 diameters, although higher reductions are feasible.
Microimages are usually generated first on silver halide film in a camera, by photographing a printed document or the face of a cathode ray tube, for example. The photographic film may be handled in roll form, typically 16 or 35 mm. in width, or in sheets referred to as microfiche. A microfiche is a sheet or card, typically about 4 .times. 6 inches, on which the recorded frames are aligned in rows and columns. The microfiche format is finding increasing favor in the field because of the convenience of organizing, filing, and retrieving desired records.
For further distribution of microfilmed records, direct duplicate copies of the original silver halide films are frequently produced. The most convenient and inexpensive means of making such duplicate copies is by contact exposure with film materials whose light-sensitive ingredient is a diazonium salt. These copy films may utilize the conventional diazo process, yielding direct positive dye images, or the vesicular process, in which the image is composed of microscopic gas bubbles, and the copy is a reversal of the original. In both cases the exposure requires radiation in the violet and near ultraviolet portion of the spectrum. A variety of equipment is available for microfilm duplication, roll to roll, roll to sheet and sheet to sheet.